Sediment Thickness and Moho Depth Beneath Western Indonesian Region From Teleseismic Receiver Function Syaiful Bahri Universitas Gadjah Mada Wiwit Suryanto ( [email protected] ) Universitas Gadjah Mada Fakultas Matematika dan Ilmu Pengetahuan Alam https://orcid.org/0000- 0002-6275-7880 Drajat Ngadmanto BMKG: Badan Meteorologi Klimatologi dan Geosika Full paper Keywords: teleseismic receiver function, sediment thickness, Moho depth, Western Indonesia Region Posted Date: March 31st, 2021 DOI: https://doi.org/10.21203/rs.3.rs-356661/v1 License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License Page 1/24 Abstract The Earth's crust layer and sediment in Western Indonesia has been studied using the inversion of teleseismic receiver function from BMKG’s seismic network. Earthquake events were analyzed in this study with a moment magnitude greater than 6.0 with epicentral distances of 30° to 90°. A total of 60 earthquake events were observed and recorded by 91 stations around the study area. Furthermore, an inversion process was carried out using the initial velocity model from the modication of the AK135f velocity model to obtain the shear wave velocity structure below each stations. The velocity model from the azimuthally stacked vertical receiver function showed that the sediment layer had a relatively medium shear wave velocity value with an average of 2.1 km/s, while the crust layer had 4.60 km/s. The sedimentary layer thickness in this region also varies between 2 km to 10 km. A relatively thick sediment layer of about 8 km to 10 km was observed in two locations, in East Kalimantan associated with the Kutai Basin and Northern part of Sumatera in the North Sumatera Basin, a two major oil producer basinal area in Indonesia. The Moho discontinuity was also found at depths that vary between 16 km to 50 km. In addition, the most shallow Moho depth is 16 km below the North Kalimantan and North part of West Java, while the deeper Moho depth of 50 km is located below East Kalimantan, Central Kalimantan, North Sumatera and South Sumatera. 1. Introduction Indonesia is part of the Eurasian plate, bordering the Indo-Australian plate in the south and the Pacic plate in the east. The tectonic mechanism divides Indonesia into two regions, namely the western and eastern parts which formed in the Mesozoic to Cenozoic era. They were formed based on the process of directional change and rate of movement of the tectonic plates. During the Jurassic to late Cretaceous period, active tectonics resulted in plate movement from the Malay Peninsula area in mainland Asia, as well as large islands such as Sumatera, Kalimantan and Java to other small islands, this plate is called the Sundaland (Hall, 2009). Sundaland tectonics is part of the Eurasian plate located in western Indonesia. The tectonic development experienced the Indo-Australian plate collision under the Sundaland plate which resulted in the Sunda Arc area that stretches from Sumatera, Java, to the island of Sumbawa (Soeria-Atmadja et al., 1998). The activity of continuous plate convergence up to the present time affects the formation of complex geological structures. Western and Eastern Indonesia are separated by the islands of Kalimantan and Sulawesi with a complex structure, which is also a transitional area between the peculiarities of the geological features of the Asian and the Australian continent. Previous studies on crustal structure have been carried out in Indonesia using the receiver function method, including in the island of Sumatera, where the thickness of the earth's crust varies from 16 km to 40 km (Kieling et al., 2011; Macpherson et al., 2012; Bora et al., 2016). Other researchers performed the receiver function study in the island of Java where the thickness of the crust in the western part of the island was 25–37 km (Syuhada and Anggono, 2016; Anggono et al., Page 2/24 2020), in the central and east part of Java island about 25–39 km (Wölbern and Rümpker, 2016; Amukti et al., 2019). Recently, the receiver function study using limited number of stations in Borneo or Kalimantan Island was carried out by Bahri et al., 2021. They showed the evidence of thickening of the Earth’s crust in cross-section from east Java to the north to Borneo Island. However, information on sediment layer thickness and the Moho depth for western Indonesia is still limited, especially in Kalimantan and Bali. In this study, the receiver function was used to model the structure under the observation station using a three component seismometer (Herrmann et al., 2001; Suryanto et al., 2010) for all available seismic stations belonging to the Meteorology, Climatology, and Geophysical Agency (BMKG) of Indonesia. This research is very important, both for the purposes of earthquake disaster mitigation and for the precise determination of earthquake hypocenter. It also establishes practical purposes in the development of oil and gas exploration in western Indonesia. 2. Geology Setting Tectonic evolution has provided an overview for the geological history of western Indonesia. This can be explained by the arrangement and age of the rocks in the area. Tectonic interactions result in complex geological conditions in the presence of troughs, non-volcanic outer arcs, volcanic, arc basins and back arcs. This study obtained geological information based on the area of the sediment basin for the islands of Sumatera, Java, Bali and Kalimantan. The formation of sedimentary basins was closely related to the movement of the crust from the tectonic processes experienced by the plates. The area of the sedimentary basin in western Indonesia is shown in Fig. 1. This map is based on gravity and geological data. The classication of basins based on age is divided into two, namely tertiary and pre-tertiary basins. Based on the tectonic arrangement, the basins are further divided into 11, namely back-arc, intermontane, fore-arc, trench, foreland, passive margin, oceanic basins, deltaic basins, rifting valley, transtensional, and transtensional marginal oceanic basin. Based on recent geology and geophysics information, Satyana et al. (2012) found about 86 basins with 16 producing oil/gas, 7 discoveries based on well data, and 25 with hydrocarbon indication from well data, 37 without current discoveries and 4 without seismic data. (Geological Agencies of Indonesia, 2009). 3. Data Set This study utilized seismogram data from the global network belonging to Indonesia’s BMKG (IA Network). Each station had a three-component broadband type seismometer sensor operated by BMKG and supported by GFZ-Potsdam Germany with 91 stations spread across Sumatera, Java, Bali and Kalimantan (Fig. 2). The earthquake events were selected with the criteria of a moment magnitude > 6.0, with an epicentral distance of 30°-90° (teleseismic event). The early stage of the processing involved the addition of header parameters to the raw data, such as the coordinates of the earthquake source information, the origin time, Page 3/24 and the magnitude of the earthquake. A further processing step included the instrument correction to produce the true ground motion in units of m/s (Herrmann, 2013). 4. Methodology In the rst step of data processing, the rotation from the cartesian coordinate system (X,Y,Z) to the radial component was performed using a simple 2D rotation of coordinate transformation, converting two horizontal components into radial (R) and transverse (T) components. The Z,R,T data were then windowed using a time window of about 10 seconds before the arrival time of the P-wave and 30 seconds after the arrival time of the P-wave, with the conversion phase of the Ps-wave appearing. Furthermore, the receiver function calculation involved an iterative time-domain deconvolution technique (Ligorría and Ammon, 1999) using a Gauss lter parameter at a width of 2.5 and a limit of 500 iterations. In this study, the response in the radial direction was selected because the Ps-wave response in Moho was seen to be the greatest in its waveform. The response receiver function in the radial direction of each earthquake was selected for the best (showing the same wave phase response in each earthquake) and then stacked altogether (Fig. 3). Receiver function inversion calculation utilized the standard receiver function code developed by Herrmann and Ammon (2004). Inversion calculations were carried out to obtain a model of the S-wave velocity below the station (Fig. 4). The inversion process was based on an initial velocity model modied from the AK135f velocity model developed by Kennett et al. (1995). Furthermore, the observed data were in form of stacks of waves at each station. As many as 15 iterations were limited until a match between the response of the prediction model and the observational data related to a minimum of their mist. 5. Results And Discussion 5.1 Receiver function response In general, the location of seismic stations in Sumatera Island lies in various basinal areas and tectonic setting. About 41 stations had a receiver function waveform with a characteristic of the P-wave at 0–0.5s (Fig. 5.a), the converted P-S at 1.0 s and 2.5 related with the sediment layer below the station, which is also called the Pss sediment phase. This station was associated with the geological environment of the sedimentary basin on the island of Sumatera. In addition, phase amplitude at 3.0 to 5.5 s were assumed to be the Ps-waves from Moho, and multiple (PpPs) indicated in signals from 12.0 to 16.0 s. In Java Island, about 34 seismic stations were observed. These stations are associated with a basinal area classication consisting of back-arc in the northern part of Java, fore-arc in the south, and Intermontane basin.